Your use case is certainly one of the ones that couldn't be easily dismissed.
But the more I've considered it, the more I feel it can't be unseen... A Rebol with FUSED! will be far ahead in usefulness beyond any Rebols without it. The dialecting powers are just too great to ignore.
In non-dialected cases... I think making FUSED! error by default and not trying to imbue it with any "non-Rebolish" behaviors, but then having people take control of their specific scenarios via RebindableSyntax makes the most sense.
This way we aren't hinging any fundamental behaviors on it... like saying block[n] is a 0-based pick whereas block.(n) is a 1-based pick (just to pick a random example).
However, if you faced a circumstance where that would be useful--or wanted to do things like what @Danny is trying with the DOM language--you could, and mix it freely with other code.
So I've Committed To Trying This
It has been one of the great unknowns of the design, and we've reached critical mass I think to where it's time to bite the bullet... space out our brackets... and see where this goes.
I'm going to put on my FUSED! hat and see what I can cook up.